The disclosure of Japanese Patent Application No. 2000-278771 filed on Sep. 13, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a gear supporting structure for supporting two gears on a common shaft and application of the structure to a hybrid driving unit and, more particularly, to an art for reducing a span for supporting two gears on a shaft and shortening axial lengths of a gear supporting structure and a hybrid driving unit employing the gear supporting structure.
2. Description of Related Art
In a power transmission device wherein powers of different origins are transmitted from two driving sources to a common shaft via gears, it is typical to adopt a structure wherein two driven gears, that constitute a pair by meshing with drive gears drivingly connected with the driving sources respectively, are axially arranged and supported in a whirl-stopping (i.e. stopping the rotation of the gears) manner. Because such a structure necessitates a span corresponding to a width of at least two driven gears on the shaft side, a problem is caused if it is adopted in a power transmission device having a shaft with a limited axial length.
In view of the problem, there is another structure wherein two drive gears mesh with a common driven gear at circumferentially different positions to reduce a span of a shaft for supporting the driven gear. As an example in which such a structure is applied to a driving unit, there is an art disclosed in Japanese Patent Application Laid-Open No. HEI 8-183347. This driving unit is a hybrid driving unit employing an engine and a motor as power sources, and transmits powers of two different origins to wheels via a counter shaft. A counter driven gear (a third gear 32 as termed in the above-mentioned publication) is meshed with a drive gear drivingly connected with the engine side (a first gear 15 as termed in the above-mentioned publication) and a drive gear drivingly connected with the motor side (a second gear 27 as termed in the above-mentioned publication), so that each of the outputs from the engine and the motor is inputted to the counter shaft at an arbitrary gear ratio.
Requirements imposed on the engine in the hybrid driving unit as described above vary quite often depending on the vehicle type, e.g., whether importance is set on fuel consumption or acceleration. While a relatively high total gear ratio from the engine to the wheels needs to be set in the former case, a relatively low total gear ratio needs to be set in the latter case. In the conventional hybrid driving unit as described above, however, if an attempt has been made to change an engine-side total gear ratio in accordance with the requirements imposed on the vehicle, diameters of the engine-side drive gear and the counter driven gear are changed. In parallel with these changes, it is also inevitable to change a diameter of the motor-side drive gear that also meshes with the counter driven gear, thus, adversely affecting a motor-side total gear ratio.
Also, from the standpoint of gear noise, in a structure wherein the engine-side drive gear and the motor-side drive gear simultaneously mesh with the counter driven gear, since meshing orders (noise frequencies) are equal to each other, gear noise of a greater level is caused. In addition, it is impossible to determine from a discrepancy between noise frequencies whether a noise-causing meshing portion is between the counter driven gear and the engine-side drive gear or between the counter driven gear and the motor-side drive gear. Therefore, no measure can be used for noise reduction.
Thus, a method in which an engine-side counter driven gear and a motor-side counter driven gear are axially disposed in parallel can be used for a generally employed structure. In general, however, a power transmission device employs helical gears to ensure that tooth flanks continuously mesh with one another, and a moment load is applied to a gear supporting portion due to a thrust force applied to the tooth flanks. Thus, a portion (hub portion) for supporting the gears on the shaft is required to have a certain axial span to inhibit the gears from being inclined with respect to the shaft due to the moment load. The axial length of the counter shaft is thus increased because of a combined increase in the number of gears and a demand for the axial span. Consequently, the total length of the hybrid driving unit itself is also increased, thus, adversely affecting the mountability to the vehicle.
The invention has been made in view of the aforementioned circumstances. The invention thus provides a gear supporting structure that eliminates eccentricity and inclination of two gears with respect to a shaft while minimizing a span for supporting the gears on the shaft. The invention further provides a hybrid driving unit which easily varies the settings of an engine-side total gear ratio and a motor-side total gear ratio if the occasion demands, allows noise reduction measures to be taken and suppresses an increase in the axial length of the counter shaft.
To achieve the foregoing, according to a first exemplary aspect of the invention, there is provided a shaft, a first gear with a whirl-stop portion engaging the shaft, a first fitting portion fitted to the shaft and a boss portion extending axially, wherein the first gear is supported by the shaft in whirl-stopping and centering manners by engaging the whirl-stop portion with the shaft and fitting the first fitting portion to the shaft, and a second gear with a second fitting portion fitted to an outer periphery of the boss portion, wherein the second gear is supported by the first gear in a centering manner by fitting the second fitting portion to the outer periphery of the boss portion, is connected with the first gear in a whirl-stopping manner by a fastening device, and is supported by the shaft in whirl-stopping and centering manners via the first gear.
In the construction according to the first exemplary aspect of the invention, since the second gear is disposed in the boss portion of the first gear, the axial span of the second gear does not contribute to the elongation of the axial length of the shaft. Accordingly, the shaft is only required to have an axial length corresponding to the span of the boss portion. That is, there is no particular need to elongate the total length of the shaft in comparison with the construction of the related art. Further, since the boss portion allows a required axial span for suppressing the inclination of the gears, this structure is also advantageous from the standpoint of gear noise. Furthermore, the first fitting portion is regarded as a portion supported at its axial center by the shaft, and the whirl-stop portion is regarded as a portion for transmitting a torque to and from the shaft. In other words, the first fitting portion and the whirl-stop portion are clearly distinguished from each other. Thus, it is possible to improve supporting precision. The second gear is supported at its axial center by the boss portion of the first gear and connected therewith by the fastening device, whereby the second gear maintains a high degree of parallelization with respect to the first gear with a short span. As a whole, the compact arrangement can be achieved.
In the first exemplary aspect, the first fitting portion and the second fitting portion may be overlapped with each other radially with respect to an axis of the shaft.
In this construction, the first fitting portion of the first gear supported at its axial center by the shaft and the second fitting portion of the second gear supported at its axial center by the first fitting portion are overlapped with each other radially with respect to the axis of the shaft, whereby supporting precision therebetween is improved. Thus, both the gears can be ensured of a high axial supporting precision for the shaft.
In the first exemplary aspect, the structure may be provided with a first counter gear meshing with the first gear and a second counter gear meshing with the second gear, wherein the second gear and the second counter gear may be helical gears having helical teeth that are twisted in such a direction that a thrust force applied to the second gear acts as a pressing force applied to the first gear when a torque is transmitted from the second gear to the shaft via the first gear with the second counter gear on the driving side.
In this construction, it is possible to reduce a load applied to the fastening device at the time of driving and thus extend a life span of the fastening device.
According to a second exemplary aspect of the invention, there is provided a a hybrid driving unit comprising an engine-side drive gear drivingly connected with an engine, a motor-side drive gear drivingly connected with a motor, a counter shaft drivingly connected with wheels, an engine-side counter driven gear that meshes with the engine-side drive gear, that is supported by the counter shaft in whirl-stopping and centering manners, and that transmits power from the engine to the counter shaft, a motor-side counter driven gear that meshes with the motor-side drive gear, that is supported by the counter shaft in whirl-stopping and centering manners, and that transmits power from the motor to the counter shaft, and an axially extending boss portion formed in one of the counter driven gears with the other counter driven gear supported by the boss portion in whirl-stopping and centering manners and supported by the counter shaft in whirl-stopping and centering manners via the one of the counter driven gears.
In the construction according to the second exemplary aspect of the invention, since the other counter driven gear is disposed in the boss portion of the one of the counter driven gears, the axial span of the other counter driven gear does not contribute to the elongation of the axial length of the shaft. Accordingly, the shaft is only required to have an axial length corresponding to the span of the boss portion, and it is only the axial length of the boss portion that is relevant to the axial length of the shaft. Thus, there is no need to elongate the total length of the hybrid driving unit in comparison with the construction of the related art. Further, since the boss portion allows a required axial span for suppressing inclination of the gears, this hybrid driving unit is also advantageous from the standpoint of gear noise.
In the second exemplary aspect, the one of the counter driven gears may have a whirl-stop portion engaging the counter shaft and a first fitting portion fitted to the counter shaft, and may be supported by the counter shaft in whirl-stopping and centering manners by engaging the whirl-stop portion with the counter shaft and fitting the first fitting portion to the counter shaft.
In this construction, the first fitting portion is regarded as a portion supported at its axial center by the shaft, and the whirl-stop portion is regarded as a portion for transmitting a torque to and from the shaft. In other words, the first fitting portion and the whirl-stop portion are clearly distinguished from each other, thus improving supporting precision.
In the second exemplary aspect, the other counter driven gear may have a second fitting portion fitted to an outer periphery of the boss portion of the one of the counter driven gears, may be supported by the one of the counter driven gears in a centering manner by fitting the second fitting portion to the outer periphery of the boss portion, may be connected with the one of the counter driven gears in a whirl-stopping manner by a fastening device, and may be supported by the counter shaft in whirl-stopping and centering manners via the one of the counter driven gears.
In this construction, the other counter driven gear is supported at its axial center by the boss portion, whereby the other counter driven gear can be ensured of a high degree of supporting precision with a short span. As a whole, compact arrangement can be achieved.
In the second exemplary aspect, the first fitting portion and the second fitting portion may be overlapped with each other radially with respect to an axis of the counter shaft.
In this construction, the first fitting portion of the one of the counter driven gears supported at its axial center by the counter shaft and the second fitting portion of the other counter driven gear supported at its axial center by the first fitting portion are overlapped with each other radially with respect to the axis of the shaft, whereby supporting precision therebetween is improved. Thus, both the gears can be ensured of a high axial supporting precision for the counter shaft.
In the second exemplary aspect, the hybrid driving unit may be installed in a vehicle, and the other counter driven gear and the drive gear meshing therewith may be helical gears having helical teeth that are twisted in such a direction that a thrust force applied to the other counter driven gear acts as a pressing force applied to the one of the counter driven gears during power running of the vehicle.
In this construction, because a greater load resulting from torque transmission is applied to the other counter driven gear during power running than during regeneration while the vehicle is running, the burden imposed on the fastening device can be reduced on the side of a greater load. Thus, it is possible to extend a life span of the fastening device.
In the second exemplary aspect, the one of the counter driven gears may be the motor-side counter driven gear, and the other counter driven gear may be the engine-side counter driven gear.
In this construction, the engine-side counter driven gear is disposed in the boss portion of the motor-side counter driven gear. Even if the requirements imposed on the vehicle vary e.g. depending on whether importance is set on fuel consumption or acceleration, nothing more than the replacement of the engine-side counter driven gear is required. Therefore, the number of variations of the driving unit can be easily increased. Further, from the standpoint of a torque inputted to the counter shaft, the torque from the motor side is also greater than the torque from the engine side during power running of the vehicle. Further, since the torque during regeneration by the motor is also inputted, the motor-side counter driven gear that requires high precision is given priority and supported on the counter shaft directly, whereas the engine-side counter driven gear is supported on the counter shaft indirectly via the boss portion. Thus, it is possible to adopt a supporting mode depending on a torque load.